This represented a milestone for an industry battling with digital transition.
Mail Online has long been the most popular newspaper site in the UK and, according to comScore figures, overtook The New York Times to become the leading online English-language newspaper at the start of the year.
According to sources, the publisher realised it had achieved operational profitability 'a month earlier than anticipated', having crunched the numbers for June, a period boosted by ad spend around the Queen's Diamond Jubilee and Euro 2012.
Stephen Daintith, finance director at DMGT, told Media-Week: 'I can confirm that Mail Online did make its first profit in June 2012 on a run rate basis. Going forward, we hope to remain profitable for the remainder of the year.
'We are expecting Mail Online's digital revenues for the year to reach between £25m and £30m.'
Last year, Mail Online's revenues were £16m.
The 'run rate basis' is an estimate of a firm's future financial performance, assuming current conditions continue.
The site's move into the black comes despite the publisher investing in expanding its US operation and providing specific content for India.